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Topic: Basic Rocket Science Q & A (Read 331782 times)

Since all objects in Earth have the same rotational speed as the Earth itself (around 1700km/h), if I wanted a sattelite in an orbit with the same orbital speed, could I launch a rocket facing totally orthogonal to the earth's surface without providing any additional orbital speed and only using the rocket to reach the needed altitude???

by the way, why arent sattelites injected directly to geosynchronous orbits?? why do they must first go to LEO and then use a transfer orbit???

It is possible but would be horribly inefficient. An orbit with a 1700 km/h circular velocity would be so high that you'd be fighting gravity losses the entire way. The current method (thrusting vertically only as much as needed to get above most of the atmosphere, gradually curving horizontally to build up to orbital velocity, then performing impulsive transfers to higher orbits) is far, far more efficient.

My question is how is that accomplished with a solid fueled upper stage, for example, a Castor 30.

Solids generally have very well predicted impulse. Therefore, the precision necessary to achieve the target orbit can be controlled by having the excess performance on lower stages and losing it during the coast before the solid ignites. This method means that the flight computer calculates the variable coast time rather than a variable burn time. I suppose you could also do it by changing the vector of the solid's burn. That might be a harder guidance problem.

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If I like something on NSF, it's probably because I know it to be accurate. Every once in a while, it's just something I agree with. Facts generally receive the former.

1) Does it (or would it) ever make sense to launch an orbit-bound rocket at an acute angle with respect to the ground?

Japan's M-V was launched that way, from a launch rail (see photo). I believe that NASA's Scout may have also been launched tilted slightly from vertical. Pegasus, of course, is nearly horizontal when dropped.

You know the rocket equations that say how much of a SSTO would have to be fuel to achieve orbit at Earth? (I think it is about 80%) Anyway, I was just wondering what percentage of a SSTO taking off from Mars would have to be fuel.

Regarding GEO perturbations: recently, I read an article in The Space Review (http://www.thespacereview.com/article/1290/1) which talked about DSP23's latest status, and explanied the East-West perturbations are mainly caused because of a pair of bulges at or close to Earth's Equator, quoted to be at 75ºE and 225ºE. I had a faint knowledge of the geoid and its characteristics, but didn't recall its geometry or the key to interpreting it, so I looked for some images, like this one:

I found that what you find at 75ºE is the major Indic Ocean perturbation, but it's a negative perturbation, meaning (and I checked this several times through different sources) it exerts less gravitational pull. The strongest pull is caused by the Indonesian anomaly. At 225ºE, we find something similar, though less obvious: a negative anomaly in the North-East Pacific. There is a positive anomaly nearby, in the Peruvian Andes. The simplest explanation for the geoid is that a ball wouldn't roll in any point in its surface, because it shows the isogravity lines (or whatever you call them)

Intuition (and simple physics) tell the satellites should be accelerated by the positive anomalies, hence causing an oscillatory motion between two points centered in the gravity well: the strongest gravity source. Are people somehow disoriented by the graphical representations of the geoid, thinking the wells are the "lowest" points in the figure? Or am I missing something entirely and being ridiculously stubborn?

I think they may have meant that the orbits get bulges at 75 and 225? Or, it may be that the Geoid is low there because the seafloor is higher density than land, and so has a larger effective gravity...

If you think the Earth has a tricky gravity field, here's a low-res grav map for the Moon; there's a reason we're sending GRAIL...

I think they may have meant that the orbits get bulges at 75 and 225? Or, it may be that the Geoid is low there because the seafloor is higher density than land, and so has a larger effective gravity...

That's why at first I thought I was reading things incorrectly, but the more I think about it, the more convinced I am they read the geoid incorrectly. Here's the exact quote from The Space Review:

Quote

In fact, there are two “bulges” along the Equator at approximately 75° and 225° East longitude. These gravity “troughs” pull satellites in geostationary orbit east or west towards whichever is closest, giving the satellite an apparent east or west drift.

The gravity vector should always be perpendicular to the surface of the geoid, so the ocean floor, even though higher density, should have lower gravitational pull (the gravity vector gets 'attracted' by the redder, higher mounds in the geoid)

The 'bulges' in the orbits you mention are a good clue as to what they may be trying to say in the article... but that would make a satellite drifting out of its GEO slot over Gabon oscillate between said point and the central Pacific, more or less. Instead of the 8-135ºE oscillation, it would move between 8 and 230-240ºE, almost twice the distance.

1) Does it (or would it) ever make sense to launch an orbit-bound rocket at an acute angle with respect to the ground?

Japan's M-V was launched that way, from a launch rail (see photo). I believe that NASA's Scout may have also been launched tilted slightly from vertical. Pegasus, of course, is nearly horizontal when dropped.

- Ed Kyle

And on a semi-related note, the launch system of the Soyuz rotates on its base in order to put the rocket in the right orientation, depending on the launch direction (azimuth). Modern rockets perform this adjustment early in flight using roll control, but this was beyond the avionics of the early R-7.

You know the rocket equations that say how much of a SSTO would have to be fuel to achieve orbit at Earth? (I think it is about 80%) Anyway, I was just wondering what percentage of a SSTO taking off from Mars would have to be fuel.

Thanks in advance

From my (very rough) calcullations, for LOX / LH2 (isp 460 sec.) it would be about 54.5%. Perhaps more realistic isp 448 sec. gives you about 55.4%. But if you'd like to use liquid methane (isp at ~380 sec.) instead of LH2, your percentage rises to 61.5%. Note that these figures don't take into account the atmosferic drag, trajectory shaping, etc.

From my (very rough) calcullations, for LOX / LH2 (isp 460 sec.) it would be about 54.5%. Perhaps more realistic isp 448 sec. gives you about 55.4%. But if you'd like to use liquid methane (isp at ~380 sec.) instead of LH2, your percentage rises to 61.5%. Note that these figures don't take into account the atmosferic drag, trajectory shaping, etc.

A more fun number would be what is the mass fraction required for SSTER (Single Stage To Earth Return)

From my (very rough) calcullations, for LOX / LH2 (isp 460 sec.) it would be about 54.5%. Perhaps more realistic isp 448 sec. gives you about 55.4%. But if you'd like to use liquid methane (isp at ~380 sec.) instead of LH2, your percentage rises to 61.5%. Note that these figures don't take into account the atmosferic drag, trajectory shaping, etc.

A more fun number would be what is the mass fraction required for SSTER (Single Stage To Earth Return)

For some launch vehicles with compartments, the air from these compartments is often replaced with nitrogen, to suppress any possible fires. Why don't they use helium, instead, which would make the launch vehicle lighter?

Too expensive. Those compartments vent to atmosphere anyway. Some materials and electronics are sensitive to helium too.

Helium is actually used in places where the temperature might be so cold (due to the presence of an LH2 tank) that the GN2 would freeze. There's at least one other application for helium purge, but I'm not feeling the gumption to google it right now to determine if it's public or proprietary.

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If I like something on NSF, it's probably because I know it to be accurate. Every once in a while, it's just something I agree with. Facts generally receive the former.

Sensitive to helium in what sense? It's supposed to be an inert gas. Diffusion problems?

Dunno exactly. Helium can go through anything, apparently weakening some metals a tiny bit. Only a problem in very low margin areas. There are some spacecraft (guidance?) boxes that don't like helium. I don't know the mechanism, just that the providers ask to make sure that their boxes aren't around any helium sources.

« Last Edit: 02/21/2009 06:20 PM by Antares »

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If I like something on NSF, it's probably because I know it to be accurate. Every once in a while, it's just something I agree with. Facts generally receive the former.